A tranche of credit never used up, at the end of the day (8)
I believe the answer is:
eventide
'the end of the day' is the definition.
(I've seen this before)
'a tranche of credit never used up' is the wordplay.
'a tranche of' indicates the answer is hidden within the clue.
'used up' shows that the letters should be reversed in order (in a down clue, letters go up).
'editneve' is hidden in the letters of 'credit never'.
'editneve' written backwards gives 'EVENTIDE'.
'at' is the link.
(Other definitions for eventide that I've seen before include "onset of dusk" , "End of the day to the poet" , "End of the day (poetic)" , "some time later?" , "latter part of the day" .)